The invention relates to a process for the isomerization of light paraffins, such as pentanes and hexanes, to produce more highly branched paraffins of higher octane number and greater utility as naphtha boiling range motor fuel. The invention specifically relates to an improvement in the fractional distillation scheme used to recover a C4 desorbent, a recycle stream and a product stream from an effluent of an adsorptive separation zone used to recover the highest octane paraffins.
The majority of the naphtha boiling range hydrocarbons recovered from petroleum do not have the high octane numbers desired for modern gasolines. For instance, straight chain or relatively straight chain C5 and C6 hydrocarbons have an octane number which is lower than desired for gasoline blending components. As a result it is necessary for modern petroleum refineries to build high octane molecules, as by alkylation, and to increase the octane of existing straight chain molecules by isomerization. By isomerizing these straight chain molecules to more highly branched molecules the octane number of the molecules is increased.
Isomerization of naphtha boiling range hydrocarbons is affected by contacting the hydrocarbons with an isomerization catalyst at isomerization conditions. Unfortunately, such isomerization steps do not result in a complete conversion of the straight chain feed molecules, and a sizable percentage of the isomerate or product of this contacting consists of molecules which have only a moderate increase in branching. In order to further increase the octane of the product it is therefore necessary to separate out and recycle the relatively less branched and therefore lower octane hydrocarbons to the isomerization zone. This separation step can be performed by fractional distillation but adsorption is more effective in performing a division between the close boiling low and high octane molecules. The subject invention relates to fractional distillation steps performed as part of an adsorptive separation used in the recovery of high octane hydrocarbons for the purpose of recycling low octane hydrocarbons to an isomerization zone.
The commercial benefit of isomerizing normal paraffins to increase their octane has led to the development of a number of separatory schemes and techniques. The integration of isomerization and adsorption is described for instance in U.S. Pat. No. 2,921,104 issued to V. Haensel and in U.S. Pat. No. 4,717,784 issued to S. C. Stem et. al. The process of this latter reference employs an adsorbent comprising a ferrierite molecular sieve to produce a normal paraffin recycle stream to an isomerization zone. Monomethyl branched paraffins are also recycled.
U.S. Pat. No. 5,026,951 issued to R. J. Schmidt et. al. illustrates a process flow comprising an isomerization zone and a simulated moving bed adsorptive separation zone for the recovery of high octane C6 hydrocarbons and the recycling of low octane hydrocarbons. The raffinate (unadsorbed) effluent of the adsorptive separation zone is passed into a raffinate column to generate a desorbent stream and the recycle stream. This reference describes the use of n-butane as a desorbent, with n-butane being removed overhead in the raffinate column and recycled to the adsorption zone. U.S. Pat. No. 5,107,052 issued to B. McCulloch et. al. also illustrates this sequential flow but employs aluminophosphate adsorbents and a C6-C10 normal paraffin desorbent, such as normal heptane.
U.S. Pat. No. 4,804,802 issued to W. E. Evans et al., also describes the separation of a hydrocarbon feed by adsorption, with normal paraffins being recycled separately from a recycle stream comprising both normal and monomethyl paraffins. Two adsorbents are employed in series flow to accomplish this. A product of more highly branched paraffins is recovered.
U.S. Pat. No. 5,042,525 issued to R. S. Haizmann et. al. presents a variation of the combined adsorption-isomerization process for producing high octane C6 hydrocarbons. The isomerization zone effluent flows into an adsorptive separation zone, and the raffinate of the adsorptive separation zone flows into a midpoint of a deisohexanizer column. Desorbent normal hexane is removed from this column as a sidecut and recycled.
The invention is an improved configuration of the fractional distillation zone used downstream of an adsorption zone which is separating paraffinic hydrocarbons. The invention reduces the capital and utility costs of this fractional distillation zone by combining two prior art columns into one column.
A broad embodiment of the invention may be characterized as a process for recovering high octane, di-branched paraffins from the raffinate stream of an adsorptive separation process, which process comprises passing a raffinate stream removed from an adsorptive separation zone, which stream comprises a desorbent hydrocarbon, mono-branched paraffins and di-branched paraffins, into a fractional distillation column maintained at fractionation conditions, with said column having an intermediate section divided into adjoining first and second vertical fractionation chambers by a substantially flow preventing vertical dividing wall, with the column also containing an upper first full diameter fractionation section located above the intermediate section and a lower second full diameter fractionation section located below the intermediate section; recovering a first product stream rich in mono-branched paraffins from the second full diameter fractionation section; allowing vapor to pass upward from the second full diameter fractionation section into the first vertical fractionation chamber, and allowing vapor to pass upward from the first vertical fractionation chamber into the first full diameter fractionation section; removing an overhead vapor stream comprising the desorbent hydrocarbon from the first full diameter fractionation section, and recovering a second product stream comprising the desorbent hydrocarbon; passing liquid comprising di-branched paraffins and the desorbent hydrocarbon downward from the first full diameter fractionation section into the second vertical fractionation chamber; recovering a second product stream comprising di-branched paraffins from a lower portion of the second vertical fractionation chamber.